Enqueue Operation in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of enqueue operation on a queue implemented with a list

What is the output of the queue after performing the enqueue operation in the following code?

DSA Python

queue = [1, 2, 3]
queue.append(4)
print(queue)

A [4]

B [4, 1, 2, 3]

C [1, 2, 3, 4]

D [1, 2, 3]

💡 Hint

Remember that enqueue adds an element to the end of the queue.

The append method adds the new element 4 to the end of the list representing the queue.

❓ Predict Output

intermediate

Output of enqueue operation on a queue implemented with collections.deque

What is the output of the queue after performing the enqueue operation in the following code?

DSA Python

from collections import deque
queue = deque([10, 20, 30])
queue.append(40)
print(list(queue))

A [10, 20, 30]

B [10, 20, 30, 40]

C [40, 10, 20, 30]

D [30, 20, 10, 40]

💡 Hint

The append method of deque adds to the right end.

The append method adds 40 to the right end of the deque, preserving order.

🔧 Debug

advanced

Identify the error in enqueue operation on a linked list queue

What error will the following code raise when trying to enqueue an element?

DSA Python

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class Queue:
    def __init__(self):
        self.front = None
        self.rear = None

    def enqueue(self, data):
        new_node = Node(data)
        if self.rear is None:
            self.front = new_node
        else:
            self.rear.next = new_node
        # Missing update of rear pointer

q = Queue()
q.enqueue(5)
q.enqueue(10)
print(q.front.data, q.rear.data)

A AttributeError: 'NoneType' object has no attribute 'data'

B None None

C TypeError: unsupported operand type(s) for +: 'int' and 'Node'

D 5 10

💡 Hint

Check if rear pointer is updated after enqueue.

The rear pointer is never updated after adding a new node, so rear remains None, causing AttributeError when accessing rear.data.

❓ Predict Output

advanced

Output of enqueue operation with circular queue array implementation

Given a circular queue of size 5 with front=3, rear=4 and elements [_, _, _, 10, 20], what is the queue state after enqueueing 30?

DSA Python

size = 5
queue = [None, None, None, 10, 20]
front = 3
rear = 4
rear = (rear + 1) % size
queue[rear] = 30
print(queue)

A [30, None, None, 10, 20]

B [None, 30, None, 10, 20]

C [None, None, 30, 10, 20]

D [None, None, None, 10, 20]

💡 Hint

Remember to wrap around using modulo for circular queue.

rear moves from 4 to 0 (wrap around), so queue[0] = 30.

🧠 Conceptual

expert

Maximum number of elements after enqueue operations in a fixed-size circular queue

A circular queue has size 7. Initially, it is empty. After performing 5 enqueue operations and 3 dequeue operations, what is the maximum number of elements that can be in the queue?

A 2

B 7

C 5

D 4

💡 Hint

Think about how many elements remain after dequeues.

After 5 enqueues, queue has 5 elements. After 3 dequeues, 2 elements are removed, so 3 remain. But maximum number after these operations can be 4 if enqueue and dequeue interleave differently.

Practice

1. What does the enqueue operation do in a queue data structure?

easy

A. Returns the size of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Adds an element to the end of the queue

Solution

1. Step 1: Understand the queue structure

   A queue works like a line where new elements join at the back.

2. Step 2: Define enqueue operation

   Enqueue means adding an element to the end of this line.

3. Final Answer:

   Adds an element to the end of the queue -> Option D

4. Quick Check:

   Enqueue = Add to end [OK]

Hint: Enqueue means add at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct syntax to enqueue an element 5 into a queue named q implemented as a Python list?

easy

A. q.enqueue(5)

B. q.push(5)

C. q.append(5)

D. q.insert(0, 5)

Solution

1. Step 1: Identify Python list method for adding at end

   Python lists use append() to add elements at the end.

2. Step 2: Match enqueue operation to list method

   Enqueue adds to the back, so q.append(5) is correct.

3. Final Answer:

   q.append(5) -> Option C

4. Quick Check:

   Enqueue in list = append() [OK]

Hint: Use append() to add at the end of a Python list [OK]

Common Mistakes:

• Using push() which is not a Python list method
• Using insert(0, 5) which adds at front
• Assuming enqueue is a built-in list method

3. What will be the output of the following code?

queue = []
queue.append(10)
queue.append(20)
queue.append(30)
print(queue)

medium

A. [30, 20, 10]

B. [10, 20, 30]

C. [10, 30, 20]

D. Error

Solution

1. Step 1: Trace each enqueue operation

   Appending 10, then 20, then 30 adds elements in order at the end.

2. Step 2: Check final queue state

   The list will be [10, 20, 30] after all appends.

3. Final Answer:

   [10, 20, 30] -> Option B

4. Quick Check:

   Appending in order = [10, 20, 30] [OK]

Hint: Appending adds elements in order at the end [OK]

Common Mistakes:

• Reversing the order of elements
• Thinking append adds at front
• Expecting an error due to empty list

4. Identify the error in the following enqueue implementation for a queue using a Python list:

def enqueue(queue, item):
    queue.insert(item)

q = [1, 2, 3]
enqueue(q, 4)
print(q)

medium

A. insert() requires two arguments: index and item

B. insert() cannot be used on lists

C. enqueue should use append() instead of insert()

D. queue is not defined

Solution

1. Step 1: Check insert() method usage

   Python list's insert() needs two arguments: position index and the item.

2. Step 2: Identify missing argument

   The code calls insert(item) with only one argument, causing an error.

3. Final Answer:

   insert() requires two arguments: index and item -> Option A

4. Quick Check:

   insert() needs index and item [OK]

Hint: insert() needs index and item arguments [OK]

Common Mistakes:

• Using insert() with one argument
• Confusing insert() with append()
• Ignoring Python method argument requirements

5. You have a queue implemented as a Python list. You want to enqueue multiple elements [7, 8, 9] at once while preserving order. Which code snippet correctly enqueues all elements in order?

hard

A. queue.extend([7, 8, 9])

B. for x in [7, 8, 9]:
queue.insert(0, x)

C. queue.append([7, 8, 9])

D. queue = [7, 8, 9] + queue

Solution

1. Step 1: Understand enqueue multiple elements

   Enqueue adds elements at the end preserving order.

2. Step 2: Analyze each option

   for x in [7, 8, 9]: queue.insert(0, x) inserts at front reversing order; queue.append([7, 8, 9]) appends list as single element; queue = [7, 8, 9] + queue prepends elements; queue.extend([7, 8, 9]) extends list adding elements in order at end.

3. Final Answer:

   queue.extend([7, 8, 9]) -> Option A

4. Quick Check:

   extend() adds multiple elements at end [OK]

Hint: Use extend() to add multiple elements at queue end [OK]

Common Mistakes:

• Using append() which nests list inside queue
• Using insert(0, x) which reverses order
• Prepending elements instead of appending